Circuit arrangement with a first semiconductor device and with a plurality of second semiconductor devices

ABSTRACT

A circuit arrangement includes a first semiconductor device having a load path and a number of second semiconductor devices. Each second semiconductor device has a control terminal and a load path between a first load terminal and a second load terminal. The second semiconductor devices have their load paths connected in series and connected in series with the load path of the first semiconductor device. Each of the second semiconductor devices has a load terminal of one of the first semiconductor device and of one of the second semiconductor devices associated thereto and a voltage limiting element coupled between the control terminal of one of the second semiconductor devices and the load terminal associated with that one of the second semiconductor devices.

This is a divisional application of U.S. application Ser. No.13/599,946, entitled “Circuit Arrangement with a First SemiconductorDevice and with a Plurality of Second Semiconductor Devices” which wasfiled on Aug. 30, 2012 (now U.S. Pat. No. 9,035,690) and is incorporatedherein by reference.

TECHNICAL FIELD

Embodiments of the present invention relate to a circuit arrangementwith a first semiconductor device such as a transistor or a diode, andwith a plurality of second semiconductor devices, such transistors, inparticular low-voltage transistors, that may be used as a powersemiconductor device.

BACKGROUND

One important aim in the development of power semiconductor devices,such as power transistors or power diodes, is to produce devices with ahigh voltage blocking capability but, nevertheless, a low on-resistance(R_(ON)) in case of a transistor and a low forward voltage drop in caseof a diode. Further, it is desired to have low losses when the powersemiconductor device changes between an on-state (blocking state) and anoff-state (conducting state).

Power transistors usually include a drift region arranged between a bodyregion and a drain region and doped lower than the drain region. Theon-resistance of a conventional power transistor is dependent on thelength of the drift region in a current flow direction and on the dopingconcentration of the drift region, wherein the on-resistance decreaseswhen the length of the drift region is reduced or when the dopingconcentration in the drift region is increased. However, reducing thelength of the region or increasing the doping concentration reduces thevoltage blocking capability.

One possible way to reduce the on-resistance of a power transistorhaving a given voltage blocking capability is to provide compensationregions in the drift region, wherein the compensation regions are dopedcomplementary to the drift region. Another possible way is to providefield plates in the drift region which are dielectrically insulated fromthe drift region and which are, for example, connected to a gate orsource terminal of the transistor. In these types of power transistors,the compensation zones or the field plates partly “compensate” dopingcharges in the drift region when the component is in its off-state. Thisallows to provide a higher doping of the drift region—which reduces theon-resistance—without reducing the voltage blocking capability.

A power diode (pin diode) usually includes a low doped drift or baseregion between a first emitter region of a first doping type and asecond emitter region of a second doping type. A power diode blocks whena voltage with a first polarity (blocking voltage) is applied betweenthe first and second emitter regions, and conducts when a voltage with asecond polarity is applied between the first and second emitter regions.In the conducting state, however, a charge carrier plasma with chargecarriers of the first and second type (p-type and n-type chargecarriers) is generated in the base region. The amount of charge carrierplasma stored in the base region is dependent on a length of the baseregion and is, therefore, dependent on the voltage blocking capability,where the amount of charge carrier plasma increases when the voltageblocking capability increases. This charge carrier plasma has to beremoved before the diode may block upon applying a blocking voltage.

Recently a new type of power semiconductor device referred to as ADR(Active Drift Region) device or ADZ (Active Drift Zone) device has beenproposed. An ADR device such as an ADRFET (Active Drift RegionField-Effect Transistor) or an ADR diode includes a first semiconductordevice such as a transistor or a diode, and a plurality of secondsemiconductor device such as transistors connected in series with thefirst semiconductor device. The second semiconductor devices form theActive Drift Region of the device and are interconnected such that theoperation states of the second semiconductor devices follow theoperation state of the first semiconductor device. That is, the secondsemiconductor devices are conducting when the first semiconductor deviceis conducting, and the second semiconductor devices are blocking whenthe first semiconductor device is blocking. The overall voltage blockingcapability of the power semiconductor device corresponds to the sum ofthe voltage blocking capabilities of the individual second semiconductordevices. That is, the individual second transistors share the overallblocking voltage applied to the power semiconductor device.

According to one approach, the second semiconductor devices in an ADRdevice are implemented as normally-on transistors such as depletionMOSFETs (Metal Oxide Field-Effect Transistors) or JFETs (Junction FieldEffect Transistors). Each of these normally on-transistors has a controlterminal (gate terminal) connected to the load terminal (drain or sourceterminal) of one of the other second transistors. Depletion MOSFETs orJFETs are voltage controlled devices that can be switched on and offthrough a control voltage applied between the control terminal and oneof the load terminals. When the second transistors are interconnected asexplained above, the control voltage of one second semiconductor devicecorresponds to the load path voltage of at least one other secondsemiconductor device. This is usually noncritical when the secondsemiconductor devices are implemented with a low voltage blockingcapability such as a voltage blocking capability of between 5V and 20V.In these devices, the maximum voltage that can be applied as a controlvoltage is in the same voltage range.

However, it may desirable to use second semiconductor devices withhigher blocking voltages such as blocking voltages of up to several 10Vor even up to several 100V in order to provide a power semiconductordevice with a voltage blocking capability of several kilovolts (kV).Usually, these semiconductor devices are not capable of withstandingcontrol voltages that are as high as several 10V or even several 100V.

There is therefore a need to provide a semiconductor device arrangementwith a first semiconductor device and with a plurality of secondsemiconductor devices that is implemented with second semiconductordevices having high blocking voltages.

SUMMARY OF THE INVENTION

One embodiment relates a circuit arrangement. The circuit arrangementincludes a first semiconductor device having a load path, and aplurality of second semiconductor devices. Each second semiconductordevice has a load path between a first load terminal and a second loadterminal and a control terminal. The second semiconductor devices havetheir load paths connected in series and connected in series to the loadpath of the first semiconductor device, and each of the secondsemiconductor devices has a load terminal of one of the firstsemiconductor device and one of the second semiconductor devicesassociated thereto. Further, a voltage limiting element is coupledbetween the control terminal of at least one of the second semiconductordevices and the load terminal associated with the at least one secondsemiconductor device.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples will now be explained with reference to the drawings. Thedrawings serve to illustrate the basic principle, so that only aspectsnecessary for understanding the basic principle are illustrated. Thedrawings are not to scale. In the drawings the same reference charactersdenote like features.

FIG. 1 illustrates a circuit arrangement with a first semiconductordevice and with a plurality of second semiconductor devices;

FIG. 2 illustrates a first semiconductor device implemented as an n-typeenhancement MOSFET;

FIG. 3 illustrates a first semiconductor device implemented as an n-typedepletion MOSFET;

FIG. 4 illustrates a first semiconductor device implemented as a p-typeenhancement MOSFET;

FIG. 5 illustrates a first semiconductor device implemented as a diode;

FIG. 6 illustrates a semiconductor device arrangement with voltagelimiting elements implemented as capacitors;

FIG. 7 illustrates a semiconductor device arrangement with voltagelimiting elements implemented as diodes;

FIG. 8 illustrates a semiconductor device arrangement with voltagelimiting elements implemented as normally-on transistors;

FIG. 9 illustrates one embodiment of one second semiconductor device;and

FIG. 10 illustrates an embodiment of a diode implemented as a FET.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawings, which form a part thereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced.

Embodiments of the present invention relate to a circuit arrangement (asemiconductor device arrangement), in particular a circuit arrangementthat has the functionality of a power semiconductor device such as apower transistor or a power diode. The circuit arrangement includes afirst semiconductor device and a plurality of second semiconductordevices. The first semiconductor device is, e.g., implemented as one ofa transistor and a diode and defines the functionality of the circuitarrangement, while the second semiconductor devices mainly define thevoltage blocking capability of the circuit arrangement.

Before explaining embodiments of the invention in detail, the basicprinciple of a circuit arrangement with a first semiconductor device andwith a plurality of second semiconductor devices is explained withreference to FIG. 1. In FIG. 1, reference character 1 denotes thecircuit arrangement. The circuit arrangement 1 includes a first loadterminal 12, a second load terminal 13, an optional control terminal 11,a first semiconductor device 2 and a plurality of second semiconductordevices 3 ₁-3 _(n). The first semiconductor device 2 has an optionalcontrol terminal 21 coupled to the control terminal 11 of the circuitarrangement, and a load path between a first load terminal 22 and asecond load terminal 23. The first load terminal 22 of the firstsemiconductor device 2 is coupled to the first load terminal 12 of thecircuit arrangement. The first semiconductor device 2 is operable toassume one of two different operation states, namely an on-state(conducting state), in which the load path 22-23 conducts a current, andan off-state (blocking state), in which the load paths 22-23 blocks.

The first semiconductor device 2 is only schematically illustrated inFIG. 1. The first semiconductor device 2, that defines the electricalbehavior of the circuit arrangement 1, can be implemented in one of manydifferent ways. According to one embodiment, the first semiconductordevice 2 can be implemented as a transistor device such as a MOSFET(Metal Oxide Semiconductor Field-Effect Transistor), a FINFET or ananotube, an IGBT (Insulated Gate Bipolar Transistor), a BJT (BipolarJunction Transistor), or a JFET (Junction Field-Effect Transistor). Eachof these different types of transistors has a drain-source path or acollector-emitter path forming the load path 22-23, and has a gateterminal or a base terminal forming the control terminal 21. Just forillustration purposes, three different types of transistors that may beused to implement the first semiconductor device 2 are illustrated inFIGS. 2 to 4.

Referring to FIG. 2, the first semiconductor device 2 may be implementedas an n-type enhancement MOSFET with a source terminal forming the firstload terminal 22, a drain terminal forming the second load terminal 23,and a gate terminal forming the control terminal 21. Referring to FIG.3, the first semiconductor device 2 may be implemented as an n-typedepletion MOSFET with a source terminal forming the first load terminal22, a drain terminal forming the second load terminal 23, and a gateterminal forming the control terminal 21. Referring to FIG. 4, the firstsemiconductor device 2 may be implemented as a p-type enhancement MOSFETwith a drain terminal forming the first load terminal 22, a sourceterminal forming the second load terminal 23, and a gate terminalforming the control terminal 21. According to a further embodiment, thatis illustrated in FIG. 5, the first semiconductor device is implementedas a diode with an anode terminal forming the first load terminal 22,and a cathode terminal forming the second load terminal 23, but withouta control terminal.

In FIG. 1 as well as in the following figures reference number “3”followed by a subscript index denotes the individual secondsemiconductor devices. Same parts of the individual second semiconductordevices, such as control terminals and load terminals, have the samereference character followed by an subscript index. For example, 3 ₁denotes a first one of the second semiconductor devices that has acontrol terminal 31 ₁ and first and second load terminals 32 ₁, 33 ₁. Inthe following, when reference is made to an arbitrary one of the secondsemiconductor devices or to the plurality of the second semiconductordevices, and when no differentiation between individual secondsemiconductor devices is required, reference numbers 3, 31, 32, 33without indices will be used to denote the second semiconductor devicesand their individual parts.

The second semiconductor devices 3 are implemented as transistor devices(transistors) in the embodiment illustrated in FIG. 1 and will bereferred to as second transistors in the following. Each of the secondtransistors 3 has a control terminal 31 and a load path between a firstload terminal 32 and a second load terminal 33. The load paths 32-33 ofthe second semiconductor devices are connected in series with each otherso that the first load terminal of one second transistor is connected tothe second load terminal of a second transistor adjacent in the seriescircuit. Further, the load paths of the second transistors 3 areconnected in series with the load path 22-23 of the first semiconductordevice 2, so that the first semiconductor device 1 and the plurality ofsecond transistors 3 form a cascode-like circuit.

Referring to FIG. 1, there are n second transistors 3, with n≧2. Fromthese n second transistors 3, a 1st second transistor 3 ₁ is arrangedclosest to first semiconductor device 2 in the series circuit with the nsecond transistors 3 and has its load path 32 ₁-33 ₁ directly connectedto the load path 22-23 of the first semiconductor device 2. An n-thsecond transistors 3 _(n) is arranged most distant to firstsemiconductor device 2 in the series circuit with the n secondtransistors 3. In the embodiment illustrated in FIG. 1, there are n=4second transistors 3. However, this is only an example, the number n ofsecond transistors 3 can be selected arbitrarily, namely dependent on adesired voltage blocking capability of the circuit arrangement 1 betweenthe first and second load terminals. This is explained in greater detailherein below.

Each of the second transistors 3 has its control terminal 31 connectedto one of the load terminals of another one of the second transistors 3or to one of the load terminals of the first transistor 2. In theembodiment illustrated in FIG. 1, the 1st second transistor 3 ₁ has itscontrol terminal 31 ₁ connected to the first load terminal 22 of thefirst transistor 2. Each of the other second transistors 3 ₂-3 _(n) havetheir control terminal 31 ₂-31 _(n) connected to the first load terminal32 ₁-32 _(n-1) of the second transistor that is adjacent in the seriescircuit in the direction of the first semiconductor device 2. Assume,for explanation purposes, that 3 _(i) is one of the second transistors 3₂-3 _(n) other than the 1st second transistor 3 ₁. In this case, thecontrol terminal 31 _(i) of this second transistor (upper secondtransistor) 3 _(i) is connected to the first load terminal 32 _(i-1) ofan adjacent second transistor (lower second transistor) 3 _(i-1). Thefirst load terminal 32 _(i-1) of the lower second transistor 3 _(i-1) towhich the control terminal of the upper second transistor 3 _(i) isconnected to is not directly connected to one of the load terminals 32_(i), 33 _(i) of this upper second transistor 3 _(i). According to afurther embodiment (not illustrated), a control terminal 31 _(i) of onesecond transistor 3 _(i) is not connected to the first load terminal 31_(i-1) of that second transistor 3 _(i-1) that is directly connected tothe second transistor 3 _(i), but is connected to the load terminal 32_(i-k) of a second transistor 3 _(i-k), with k>1, farther away from thetransistor. If, for example, k=2, then the control terminal 31 _(i) ofthe second transistor 3 _(i) is connected to the first load terminal 32_(i-2) of the second transistor 3 _(i-2) that is two second transistorsaway from the second transistor 3 _(i) in the direction of the firsttransistor 2 in the series circuit.

In the embodiment illustrated in FIG. 1, the second transistors 3 arenormally-on (depletion) transistors. However, this is only an example.Each of the first semiconductor device 2 and the second transistors 3can be implemented as a normally-on transistor or as a normally-offtransistor. The individual transistors can be implemented as n-typetransistors or as p-type transistors. Implementing the secondtransistors 3 as MOSFETs is only an example. Any type of transistor canbe used to implement the first semiconductor device 2 and the secondtransistors 3, such as a MOSFET, a MISFET, a MESFET, an IGBT, a JFET, aFINFET, a nanotube device, an HEMT, etc. Independent of the type ofdevice used to implement the first semiconductor device 2 and the secondsemiconductor devices 3, these devices are connected such that each ofthe second semiconductor devices 3 is controlled by the load pathvoltage of at least one other second semiconductor devices 3 or by theload path voltage of the first semiconductor device 2.

Just for illustration purposes, the individual second transistors 3 areimplemented as depletion MOSFETs in the embodiment of FIG. 1. Each ofthe MOSFETs 3 has a gate terminal as a control terminal 21, 31, a sourceterminal as a first load terminal 22,32, and a drain terminal as asecond load terminal 23, 33. MOSFETs are voltage controlled devices thatcan be controlled by the voltage applied between the gate and sourceterminals (the control terminal and the first load terminal). Thus, inthe arrangement illustrated in FIG. 1, the 1st second transistors 3 ₁ iscontrolled through a voltage that corresponds to the load path voltageV2 of the first semiconductor device 2, and the other second transistors3 _(i) are controlled through the load path voltage of at least onesecond transistor 3 _(i-1) or 3 _(i-2). The “load path” voltage of oneMOSFET is the voltage between the first and second load terminals (drainand source terminal) of this MOSFET.

The operating principle of the circuit arrangement 1 of FIG. 1 isexplained in the following. Just for explanation purposes it is assumedthat the first semiconductor device 2 is implemented as a transistor,that the second transistors 3 are implemented as n-type depletionMOSFETs or n-type JFETs, and that the individual devices 2, 3 areinterconnected as illustrated in FIG. 1. The basic operating principle,however, also applies to semiconductor device arrangements implementedwith other types of first and second semiconductor devices. Further, itis assumed that the circuit arrangement 1 is connected in series with aload Z and that the series circuit with the load Z and the circuitarrangement is connected between supply terminals where a supply voltageV1 is available.

A depletion MOSFET and a JFET are similar in terms of their electricalbehavior. A depletion MOSFET conducts when a drive voltage (gate-sourcevoltage) is applied that is above a predefined threshold voltage. In ann-type depletion MOSFET, the threshold voltage is negative so that theMOSFET conducts when a drive voltage of about zero is applied. TheMOSFET blocks when the drive voltage decreases to below the negativethreshold voltage, that is when the drive voltage is more negative thanthe threshold voltage. The “drive voltage” is the voltage between thegate terminal and the source terminal of the device.

An n-type JFET conducts when a drive voltage is applied that is morepositive than a negative pinch-off voltage. Thus, the n-type JFETconducts when a drive voltage of about zero is applied. The JFET blockswhen the drive voltage decreases to below the pinch-off voltage, that iswhen the drive voltage is more negative than the pinch-off voltage.While in an n-type depletion MOSFET or n-type JFET the threshold voltageand the pinch-off voltage, respectively, is a negative voltage, thethreshold voltage (pinch-off voltage) is a positive voltage in a p-typedepletion MOSFET or JFET.

In the circuit arrangement of FIG. 1, the 1st second transistor 3 ₁ iscontrolled by the negative load path voltage V2 of the firstsemiconductor device 2, the 2nd second transistor is controlled by thenegative load path voltage V3 ₁ of the 1st second transistor device 3 ₁,etc. That is: VG3 ₁=−V2, VG3 ₂=−V3 ₁, etc., where VG3 ₁, VG3 ₂ are thegate-source voltages of the second transistors 3 ₁, 3 ₂. For explanationpurposes it is assumed that the first semiconductor device 2 is in anoff-state and that the magnitude of the load path voltage V2 of thefirst semiconductor device 2 is high enough to switch off (keep in anoff-state) the 1st semiconductor device 3 ₁. The 1st second transistordevice 3 ₁ then switches off (keeps in an off-state) the 2nd secondtransistor device 3 ₂, and so on. The number of second transistors thatswitch off is dependent on the supply voltage V1 applied to the seriescircuit. As the voltage across the circuit arrangement reaches thesupply voltage, a voltage across the load Z becomes zero.

When the first semiconductor device 2 is switched on by applying asuitable drive potential to the control terminal 11, the load pathvoltage V2 of the first semiconductor device V2 decreases. As this loadpath voltage V2 (more precisely: as the negative load path voltage—V2)reaches the threshold voltage of the 1st second transistor 3 ₁ the 1stsecond transistor 3 ₁ starts to conduct so that the load path voltage V3₁ of the 1st second transistor 3 ₁ decreases. Consequently, the secondtransistor 3 ₂ controlled by the load path voltage V3 ₁ of the 1stsecond transistor 3 ₁ also starts to conduct, etc. In other words, thefirst semiconductor device 2 and each of the second transistors 3 arefinally conducting so that the circuit arrangement 1 is in an on-state.

When the circuit arrangement 1 is in an on-state and when the firstsemiconductor device 2 is switched off, the voltage drop across the loadpath of the first semiconductor device 2 increases, so that the 1stsecond transistor 3 ₁ starts to switch off when the negative load-pathvoltage—V2 becomes more negative than the threshold voltage of the 1stsecond transistor 3 ₁. When the 1st second transistor 3 ₁ is switchedoff, the voltage drop across its load path increases so that the 2ndsecond transistor 3 ₂ is switched off, which in turn switches off the3rd second transistor, and so on, until each of the second transistors 3is switched off and the semiconductor device arrangement 1 is finally ina stable off-state. The external voltage applied between the second andfirst terminals 13 and 12 switches as many 2nd transistors from theon-state to the off-state as required to distribute the external voltageover the first semiconductor device 2 and the second transistors 3. Whenapplying a low external voltage, some second transistor are still in theon-state, while others are in the off-state. The number of secondtransistors that are in the off-state increases as the external voltageincreases. Thus, when a high external voltage is applied, that is in therange of the voltage blocking capability of the overall semiconductordevice arrangement, the first semiconductor device and each of thesecond transistors are in the off-state.

When the circuit arrangement 1 is in an off-state and when the firstsemiconductor device 2 is switched on, the voltage drop across the loadpath of the first transistor 2 decreases so that it switches on the 1stsecond transistor 3 ₁, which in turn switches on the 2nd secondtransistor 3 ₂, and so on. This continues until each of the secondtransistors 3 is again switched on.

The switching states of the second transistors 3 connected in serieswith the first semiconductor device 2 are dependent on the switchingstate of the first semiconductor device 2 and follow the switching stateof the first semiconductor device 2. Thus, the switching state of thesemiconductor arrangement 1 is defined by the switching state of thefirst semiconductor device 2. The circuit arrangement 1 is in anon-state (conducting state) when the first semiconductor device 2 is inan on-state, and the circuit arrangement 1 is in an off-state (blockingstate) when the first semiconductor device 2 is in an off-state.

The semiconductor arrangement 1 has a low resistance between the firstand second load terminals 12, 13 when it is in an on-state, and has ahigh resistance between the first and second load terminals 12, 13 whenit is in an off-state. In the on-state, an ohmic resistance between thefirst and second load terminals 12, 13 corresponds to the sum of theon-resistances R_(ON) of the first semiconductor device 2 and the secondtransistors 3. A voltage blocking capability, which is the maximumvoltage that can be applied between the first and second load terminals12, 13 when the circuit arrangement is in an off-state before anAvalanche breakthrough sets in, corresponds to the sum of the voltageblocking capabilities of the first semiconductor device 2 and the secondtransistors 3.

The voltage blocking capability and the on-resistance of the circuitarrangement 1 are defined by the voltage blocking capabilities of thefirst transistor 2 and the second transistors 3 and by theon-resistances of the first transistor 2 and the second transistors 3,respectively. When the arrangement 30 with the second transistors 3 isimplemented with significantly more than two second transistors 3(n>>2), such as more than 5, more than 10, or even more than 20 secondtransistors 3, the voltage blocking capability and the on-resistance ofthe circuit arrangement 1 are mainly defined by the arrangement 30 withthe second transistors 3. The overall circuit arrangement 1 can beoperated like a conventional power transistor when the firstsemiconductor device 2 is implemented as a transistor. In a conventionalpower transistor, an integrated drift region mainly defines theon-resistance and the voltage blocking capability. Thus, the arrangement30 with the second transistors 3 has a function that is equivalent tothe drift region in a conventional power transistor. The arrangement 30with the second transistors 30 can, therefore, be referred to as activedrift region (ADR) or active drift zone (ADZ). The overall semiconductordevice arrangement 1 of FIG. 1 can be referred to as ADZ transistor orADR transistor (ADZ transistor) or as ADRFET (ADZFET), when the firstsemiconductor device is implemented as a MOSFET.

When the first semiconductor device 2 is implemented as a diode, theoperating principle of the circuit arrangement 1 corresponds to theoperating principle of a power diode. In this case, the circuitarrangement 1 conducts when a voltage is applied between the first andsecond load terminals 11, 12 that forward biases the diode 2. Theforward biased diode 2 switches on the 1st second transistors 3 ₁ whichswitches on the 2nd second transistor 3 ₂, and so on. The circuitarrangement blocks when a voltage is applied between the first andsecond load terminals 11, 12 that reverse biases the diode 2. Thereverse biased diode switches off the 1^(st) second transistors 3 ₁which switches off the 2nd second transistor 3 ₂, and so on. Whenimplementing the second transistor arrangement 30 with n-type depletionMOSFETs or n-type JFETs the cathode terminal 23 of the diode 2 (see FIG.5) is connected to the source terminal of the 1st second transistor 3 ₁.In a circuit arrangement 1 with a diode as the first semiconductordevice 2 and with a plurality of MOSFETs or JFETs as second transistorsa lower amount of charge carrier plasma is stored than in a conventionalpower diode. Thus, the reverse recovery behavior of a circuitarrangement 1 with a diode as the first semiconductor device 2 and witha plurality of MOSFETs or JFETs as second transistors 3 is improvedrelative to the reverse recovery behavior of a conventional power diode.

When the semiconductor device arrangement 1 is in an off-state(blocking), the voltage applied between the first and second loadterminals 12, 13 is distributed such that a part of this voltage dropsacross the load path 22-23 of the first transistor 2, while other partsof this voltage drop across the load paths of the second transistors 3.However, there may be cases in which there is no equal distribution ofthis voltage to the second transistors 3. Instead, those secondtransistors 3 that are closer to the first semiconductor device 2 mayhave a higher voltage load than those second transistors 3 that are moredistant to the first semiconductor device 2.

In order to more equally distribute the voltage to the secondtransistors 3, the circuit arrangement optionally includes voltagelimiting means 10 ₁-10 _(n) that are configured to limit or clamp thevoltages across the load paths of the second transistors 3. Optionally,a clamping element 10 ₀ is also connected in parallel to the load path(between the source and drain terminals) of the first semiconductordevice 2. These voltage clamping means 10 ₀-10 _(n) can be implementedin many different ways. Just for illustration purposes the clampingmeans 10 ₀-10 _(n) illustrated in FIG. 1 includes Zener diodes 10 ₀-10_(n), with each Zener diode 10 ₀-10 _(n) being connected in parallelwith the load path of one of the second transistors 3 and, optionally,the first transistor 2.

Instead of the Zener diodes 10 ₀-10 _(n), tunnel diodes, PIN diodes,avalanche diodes, or the like, may be used as well. According to afurther embodiment (not illustrated), the individual clamping elements10 ₀-10 _(n) are implemented as transistors, such as, for example,p-type MOSFETs when the second transistors 3 are n-type MOSFETs. Each ofthese clamping MOSFETs has its gate terminal connected to its drainterminal, and the load path (the drain-source path) of each MOSFET isconnected in parallel with the load path of one second transistor 3.Each of the individual clamping elements, such as the Zener diodes 10₀-10 _(n) illustrated in FIG. 1, can be integrated in the samesemiconductor body as the semiconductor device (the first semiconductordevice 2 or the second transistors) it is coupled thereto.

In the circuit arrangement of FIG. 1, the first semiconductor device 1and the individual second transistors may have relatively low voltageblocking capabilities compared such as voltage blocking capabilities ofbetween 3V and 50V. However, dependent on the number n of secondtransistors 3 a high overall voltage blocking capability of up toseveral 100V, such as 600V or more, can be obtained. In some fields ofmodern power electronics power semiconductor devices with voltageblocking capabilities of several kV (kilovolts) such as 5 kV, 6 kV, or 7kV are needed. In order to implement a circuit arrangement 1 with apower transistor functionality or a power diode functionality and veryhigh voltage blocking capabilities in the range of several kV it may bedesirable to implement the second transistor arrangement 30 with secondtransistors 3 having voltage blocking capabilities higher than several10V, such as voltage blocking capabilities of several 100V. For example,a circuit arrangement with a voltage blocking capability of 6.5 kV couldbe implemented with a second transistor arrangement 30 having n=10second transistors 3 each having a voltage blocking capability of 600Vconnected in series and with a first transistor 2 having a voltageblocking capability of at least 500V.

However, each of the second transistors 3 is controlled by the load pathvoltage of the first semiconductor device 2 or the load path voltage V3of another second transistor 3, so that the drive voltage of each of thesecond transistors 3 corresponds to the load path voltage of the firstsemiconductor device 2 or to the load path voltage of another one of thesecond transistors 3, respectively. Usually, transistors having avoltage blocking capability of several 100V such as 400V, 500V, 600V, ormore, are not capable of withstanding a corresponding voltage betweenthe drive terminals, which are the gate and source terminals in aMOSFET. Conventional transistors having a voltage blocking capability ofseveral 100V are only capable to withstand voltages or several 10Vbetween the drive terminals. For example, a conventional depletionMOSFET with a voltage blocking capability of 600V is only capable towithstand a gate-source voltage of between 20V and 30V. Thus, highvoltage transistors cannot be interconnected in the transistorarrangement 30 in the same way as low voltage transistors. That is,second transistors 3 implemented as high voltage transistors cannot beinterconnected as illustrated in FIG. 1 without facing the risk ofhaving the second transistors 3 damaged. Instead, voltage protectionmeans are required that protect the control terminals of the secondtransistors 3. Different embodiments of circuit arrangements includingsuch voltage protection means are explained below.

FIG. 6 illustrates a circuit arrangement that is based on the circuitarrangement of FIG. 1. In the following, only the differences betweenthe circuit arrangement of FIG. 6 and the circuit arrangement of FIG. 1are explained, so that for those features that are not explicitlyexplained in the following the explanation provided with reference toFIG. 1 applies accordingly. In the circuit arrangement of FIG. 6, thecontrol terminals 31 of the individual second transistors 3 are coupledto the associated load terminals via voltage limiting elements 41 ₁-41_(n). The “associated load terminal” of one second transistor 3 is theload terminal the control terminal 31 of the second transistor 3 iscoupled to. That is, the load terminal associated with the controlterminal 31 ₁ of the 1st second transistor 3 ₁ is the first loadterminal 22 of the first semiconductor device 2, the load terminalassociated with the control terminal 31 ₂ of the 2nd second transistor 3₂ is the first load terminal 32 ₁ of the 1^(st) second transistor 3 ₁,and so on. In the embodiment of FIG. 6, the voltage limiting elements 41₁-41 _(n) are implemented as capacitors connected between the controlterminals 31 and the associated load terminals. Each of the secondtransistors 3 has an internal gate-source capacitance between the gateterminal (control terminal 31) and the first load terminal (sourceterminal 32). These gate-source capacitances are also illustrated inFIG. 6. In the following, reference character 41 denotes an arbitraryone of the capacitors, VG3 denotes the gate-source voltage of the secondtransistor 3 the capacitor 41 is connected to, and V3 denotes the loadpath voltage of the second transistor 3.

Referring to FIG. 6, the capacitor 41 connected between the controlterminal 31 and the associated load terminal and the internalgate-source capacitance form a capacitive voltage divider. Unlike thecircuit arrangement of FIG. 1 the drive voltage VG3 of the individualsecond transistors 3 does not correspond to the load voltage, but isonly a share of the load voltage. For example, the drive voltage(gate-source voltage) VG3 ₂ of the 2nd second transistor 3 ₂ is a shareof the load voltage V3 ₁ of the 1st second transistor 3 ₁ and is definedby the capacitance ratio of the capacitor 41 ₂ that couples the controlterminal 31 ₂ to the load terminal 32 ₁ and the gate-source capacitanceas follows:

$\begin{matrix}{{{{VG}\; 3_{2}} = {{\frac{C\; 41_{2}}{{{CG}\; 3_{2}} + {C\; 41_{2}}} \cdot V}\; 3_{1}}},} & (1)\end{matrix}$where VG3 ₂ is the gate-source voltage, C41 ₂ is the capacitance of thecapacitor 41 ₂ and CG3 ₂ is the capacitance of the gate-sourcecapacitance of the second transistor 3 ₂. Thus, by suitably adjustingthe capacitance of the capacitor 41 to the gate-source capacitance ofthe second transistor 3 the gate-source voltage VG3 can be limited.Assume, e.g., that V3 ₁=600V is the maximum load path voltage (thevoltage blocking capability) of the 1^(st) second transistor 3 ₁ andthat VG3 ₂=20V is the maximum gate-source voltage of the transistor 3 ₂.In this case, the gate-source voltage can be limited by selecting thecapacitance C41 ₂ of the capacitor 41 ₂ to be about 1/30·CG3 ₂.

According to one embodiment, the individual second transistors 3 are SiCJFETs or SiC depletion MOSFETs with a voltage blocking capabilities ofabout 600V, or more. Alternatively gallium nitride (GaN) normally-onHEMTs can be used. The individual second transistors 3 may beimplemented in one common semiconductor body. However, it is alsopossible to implement the individual second transistor 3 in differentsemiconductor bodies. According to one embodiment, the firstsemiconductor device 2 is implemented as a MOSFET, specifically as ann-type or p-type enhancement MOSFET. This first transistor device 2 mayhave a voltage blocking capability lower than the voltage blockingcapability of the second transistors 2, or may have a voltage blockingcapability corresponding to the voltage blocking capabilities of thesecond transistors 2. According to one embodiment, the voltage blockingcapability of the first transistor device 2 is 100V or less. The firstsemiconductor device 2 may be implemented based on silicon (Si), siliconcarbide (SiC), gallium nitride (GaN) or another conventionalsemiconductor material.

FIG. 7 illustrates a further embodiment of a circuit arrangement thatincludes a first semiconductor device 2 and a plurality of secondsemiconductor devices 3 connected in series with the first semiconductordevice 2. In the circuit arrangement of FIG. 7, voltage limitingelements connected between the control terminals 31 of the individualsecond transistors 3 and the associated load terminals are implementedas rectifier elements, specifically as diodes 42 ₁-42 _(n). In theembodiment of FIG. 7 in which the individual second transistors 3 areimplemented as n-type normally-on transistors, a cathode terminal ofeach diode 42 ₁-42 _(n) is connected to the control terminal 31 of thesecond transistor 3, while the anode terminal is connected to theassociated load terminal. The operating principle of the circuitarrangement of FIG. 7 is explained with reference to the diode 42 ₁connected between the control terminal 31 ₁ of the second transistor 3 ₁and the first load terminal 22 of the first semiconductor device 2. Forexplanation purposes it is assumed that the circuit arrangement 1 is inan on-state (conducting state) so that a magnitude of the load pathvoltage V2 is too low to switch off the second transistor 3 ₁. As theload path voltage V2 increases the voltage across the series circuitwith the gate-source capacitance and the diode 42 ₁ increases. The diode42 ₁ has an internal pn-junction forming a junction capacitor with acapacitance that is dependent on a reverse biasing voltage applied tothe diode 42 ₁. The capacitance of the junction capacitor decreases asthe magnitude of the voltage that reverse biases the diode 42 ₁increases. Thus, the diode 42 ₁ and the gate-source capacitance of the1st second transistor 3 ₁ form a capacitive voltage divider. As thefirst semiconductor device 2 blocks, the load path voltage V2 of thefirst semiconductor device 2 reverse biases the diode 42 ₁. At low loadvoltages V2, the capacitance of the internal junction capacitor of thediode 42 ₁ is relatively large so that (in accordance with equation (1)the load voltage mainly drops across the gate-source capacitor causingthe 1st second transistor to block. As the load voltage furtherincrease, the voltage across the diode 42 ₁ increases, so that thecapacitance of the junction capacitor decreases. This causes the diode42 to take an increasing share of the overall load voltage V2. Theoperating principle of the other diodes 42 ₂-42 _(n) that are controlledby load path voltages V3 of the second transistors 3 ₁-3 ₃ isequivalent.

A further embodiment of a circuit arrangement with second transistors 3and voltage limiting elements is illustrated in FIG. 8. In FIG. 8, theindividual voltage limiting elements 43 ₁-43 _(n) are implemented asnormally on-transistors such as JFETs or depletion MOSFETs. In theembodiment of FIG. 8, the individual voltage limiting elements 43 ₁-43_(n) are depletion FETs. However, this is only an example. The voltagelimiting elements could be implemented as any normally-on FETs as well.Each voltage limiting element has a load path (gate-source path)connected between the control terminal 31 of one second transistor 3 andthe associated load terminal. Further, a control terminal (gateterminal) is connected to the first load terminal (source terminal) 32of the second transistor it is associated with. The voltage limitingtransistor 43 is controlled by the same voltage as the second transistor3 it is associated to. The operating principle is explained withreference to the transistor 43 ₁ of the second transistor 3 ₁ in thefollowing.

For explanation purposes it is assumed that the circuit arrangement isin an on-state (conducting state) and that the load path voltage V2 ofthe first semiconductor device 2 is too low to switch off the secondtransistor 3 ₁. As the load path voltage V2 increases (when, e.g., thefirst transistor device 2 switches off) the transistor 43 ₁ isconducting at low load path voltages V2 and allows the gate-sourcecapacitance of the second transistor 3 ₁ to be charged. When the loadpath voltage V2 reaches the pinch off voltage of the transistor 43 ₁ thetransistor 43 ₁ switches off and prevents the gate-source voltage VG3 ₁of the second transistor 3 ₁ from increasing further. In the embodimentof FIG. 8, the depletion MOSFETs are p-type MOSFETs having their sourceterminal connected to the gate control terminal of the associated secondtransistor 2 and having their drain terminal connected to the first loadterminal (source terminal) of another second transistor 2 or the firstsemiconductor device, respectively.

Referring to FIG. 9, that schematically illustrates one transistor 3, ofthe second transistors 3 ₁-3 _(n), each of the second transistors 3 ₁-3_(n) may be implemented with a plurality of transistors 31_1 _(i), 31_m_(i) that have their load paths connected in parallel and that havetheir control terminals coupled to a common control terminal 31 _(i).Only one voltage limiting element, such as a capacitor (see FIG. 6), adiode (see FIG. 7) or a transistor (see FIG. 8) is required to protectone of these second transistors with a plurality of transistorsconnected in parallel.

In the embodiment of FIG. 7, the individual diodes 42 may each beimplemented as a transistor, in particular as a field-effect transistoroperating like a diode. A field effect transistor operating like a diodehas its drain terminal connected to its gate terminal. FIG. 10illustrates one embodiment of a JFET, specifically of an n-type JFET,operating like a diode. The drain terminal of the JFET is connected tothe gate terminal. The drain terminal forms an anode and the sourceterminal forms a cathode of the diode implemented by the JFET. Insteadof a JFET, any other type of FET (Field-Effect Transistor) may be usedas well.

Although various exemplary embodiments of the invention have beendisclosed, it will be apparent to those skilled in the art that variouschanges and modifications can be made which will achieve some of theadvantages of the invention without departing from the spirit and scopeof the invention. It will be obvious to those reasonably skilled in theart that other components performing the same functions may be suitablysubstituted. It should be mentioned that features explained withreference to a specific figure may be combined with features of otherfigures, even in those cases in which this has not explicitly beenmentioned. Further, the methods of the invention may be achieved ineither all software implementations, using the appropriate processorinstructions, or in hybrid implementations that utilize a combination ofhardware logic and software logic to achieve the same results. Suchmodifications to the inventive concept are intended to be covered by theappended claims.

Spatially relative terms such as “under,” “below,” “lower,” “over,”“upper” and the like, are used for ease of description to explain thepositioning of one element relative to a second element. These terms areintended to encompass different orientations of the device in additionto different orientations than those depicted in the figures. Further,terms such as “first,” “second,” and the like, are also used to describevarious elements, regions, sections, etc. and are also not intended tobe limiting. Like terms refer to like elements throughout thedescription.

As used herein, the terms “having,” “containing,” “including,”“comprising” and the like are open ended terms that indicate thepresence of stated elements or features, but do not preclude additionalelements or features. The articles “a,” “an,” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise.

It is to be understood that the features of the various embodimentsdescribed herein may be combined with each other, unless specificallynoted otherwise.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. A circuit arrangement, comprising: a firstsemiconductor device having a load path; a plurality of secondsemiconductor devices, each second semiconductor device having a controlterminal and a load path between a first load terminal and a second loadterminal, the second semiconductor devices having their load pathsconnected in series and connected in series with the load path of thefirst semiconductor device, wherein a load terminal of a first one ofthe second semiconductor devices is connected to a first load terminalof an adjacent one of the second semiconductor devices; and a voltagelimiting element coupled between the control terminal of the first oneof the second semiconductor devices and the second load terminal of theadjacent one of the second semiconductor devices, wherein the voltagelimiting element comprises a normally-on transistor.
 2. The circuitarrangement of claim 1, wherein the normally-on transistor comprises oneof a JFET, a depletion MOSFET, and a FINFET.
 3. The circuit arrangementof claim 1, wherein the normally-on transistor comprises one of a SiCJFET, a SiC depletion MOSFET, and a GaN HEMT.
 4. The circuit arrangementof claim 1, wherein the normally-on transistor comprises a load pathconnected between the control terminal of the first one of the secondsemiconductor devices and the second load terminal of the adjacent oneof the second semiconductor devices, and a control terminal coupled tothe first or second load terminals of the first one of the secondsemiconductor devices.
 5. The circuit arrangement of claim 1, whereinthe voltage limiting element comprises a depletion MOSFET.
 6. Thecircuit arrangement of claim 5, wherein: the second semiconductordevices comprise JFET transistors; and the first semiconductor devicecomprises an enhancement MOSFET.
 7. The circuit arrangement of claim 5,wherein: the second semiconductor devices comprise depletion MOSFETtransistors; and the first semiconductor device comprises an enhancementMOSFET.
 8. The circuit arrangement of claim 1, wherein the firstsemiconductor device comprises a depletion MOSFET.
 9. The circuitarrangement of claim 1, wherein the first semiconductor device comprisesa diode.
 10. The circuit arrangement of claim 1, wherein the voltagelimiting element is one of a plurality of voltage limiting elements,each voltage limiting element being coupled between the control terminalof one of the second semiconductor devices and the second load terminalof an adjacent one of the second semiconductor devices.
 11. A circuitarrangement, comprising: a first semiconductor device having a loadpath; a plurality of second semiconductor devices, each secondsemiconductor device having a control terminal and a load path between afirst load terminal and a second load terminal, the second semiconductordevices having their load paths connected in series and connected inseries with the load path of the first semiconductor device, wherein aload terminal of a first one of the second semiconductor devices isconnected to a first load terminal of an adjacent one of the secondsemiconductor devices; and a voltage limiting element coupled betweenthe control terminal of the first one of the second semiconductordevices and the second load terminal of the adjacent one of the secondsemiconductor devices, wherein the voltage limiting element comprises adepletion MOSFET.
 12. The circuit arrangement of claim 11, wherein: thesecond semiconductor devices comprise JFET transistors; and the firstsemiconductor device comprises an enhancement MOSFET.
 13. The circuitarrangement of claim 11, wherein: the second semiconductor devicescomprise depletion MOSFET transistors; and the first semiconductordevice comprises an enhancement MOSFET.